Daniel Defoe and English grammar

I’ve been reading some Defoe novels recently, and it’s surprising how many ‘mistakes’ in grammar he makes that are frequently seen these days and attributed to the poor teaching of English over the last half century: confusion of who and whom; using I in contexts that clearly demand me; and these sort of … are three that I particularly remember. The editions that I’ve been reading are all reputable ones, so I don’t think we can attribute these to printers’ errors. For those in any doubt of the antiquity of these ‘mistakes’, Defoe died in 1731.

What are we to make of this? I think we have to say that at all stages in the life of any language there are points of grammar that cause problems for even the best writers. Defoe came of a fairly well-to-do family, and received a good education, so his ignorance cannot be ‘blamed’. Since the loss of case as a regular feature of English, the remnants, in the form of the personal pronouns, have been under threat in all but the most obvious contexts, and the phrase ‘sort of’ seems to behave as a compound adjective rather than anything else, leaving the determiner ‘these’ to agree in number with the following noun (which is usually plural).